1. Field of Invention
This invention relates to color cathode ray tubes, and more particularly to color cathode ray tubes for providing excitation of a plurality of color phosphor groups on a video screen with a single electron gun whose electron beam is maintained in phosphor registration without the use of any post deflection apparatus.
2. Description of the Prior Art
One of the major problems in providing a high color fidelity video display is ensuring the registration of an exciting electron beam with each of the plurality of color phosphors on the video screen of the CRT. At the present time there are two electron beam positioning methods used to provide color registration, namely; the use of a three electron gun assembly in combination with a high precision shadow mask, and a single, or multiple, electron gun assembly with an electrically biased matrix of grid wires disposed in close proximity to the video screen. The shadow mask and grid matrix assemblies, generically called post deflection apparatus, are high cost items requiring precision tolerances in manufacture, and complicated electronic circuitry and alignment testing procedures to guarantee precise color registration, or color convergence, of the electron beam on the individual color phosphors deposited across the CRT screen surface. Furthermore, these post deflection apparatus disposed within the CRT between the gun assemblies and the video screen, are in the direct path of the deflected electron beam causing attenuation of the electron beam current density by the solid portions of the assemblies such that only twenty to thirty-five percent of the emitted beam current impinges the screen and is actually used for phosphor excitation. As a result, higher operating voltage potentials than that used in black and white CRT's are used to compensate for the loss and provide sufficient current excitation of the phosphors. This higher power requirement for color CRT's results in the use of a higher energy beam requiring higher power beam deflection systems and increased x-ray emission from the CRT, which further results in excessive power consumption and potential health hazards due to excessive radiation emission. Alternatively, a reduction of the operating voltage potentials to levels comparable with that used in balck and white CRT's, results in an unacceptable reduction in video display brilliance.